Add url to error messages

This is just a real small change, adding the URL of the request to the messages produced by `OAuth2::Error` in order to facilitate debugging.  I did not write and specs, since the `OAuth2::Error` class does not have any specific to itself, and the code changes are fully covered.

There is also a commit for ignoring `.ruby-version`, the file used by `rbenv`.

Please let me know if you would like me to make any additional changes for this pull request.  I would really like to see this pulled in.  Thanks!